Microstructures of YBCO prepared by a "grain-coating" technique were observed through transmission electron microscopy. The coating material of Ag or Bi2O3 is detected by EDS analysis only at grain boundaries and not inside grains in which abundant dislocations are found. Most of the dislocations are dissociated into partial dislocations with stacking faults. Two types of stacking fault are observed: One has a large width and is terminated by straight or smooth partial dislocations, the other is terminated by curved dislocations of which the separation is alternately large and small. The stacking fault vector is found to be 1 6, being consistent with an extra CuO layer in the 123 matrix. The coating material improves the weak-link at grain boundaries and increases the Jc, while plenty of dislocations associated with stacking faults act as effective pinning centers and contribute to increasing the Jc in a high magnetic field.
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